Optical system with angular compensator

ABSTRACT

An electro-optical projection system includes a polarizer, a reflective light modulator, an analyzer, and a compensation element. The polarizer polarizes an illumination beam to form a polarized illumination beam. The reflective light modulator receives the polarized illumination beam along a first optical path, modulates the polarized illumination beam to form an imaging beam, and reflects the imaging beam along a second optical path. The compensation element is disposed in the polarized illumination beam and the imaging beam to compensate for polarization aberrations resulting from the birefringent property of the material of the reflective light modulator. The compensation plate has a retardation generally equal in magnitude and opposite in sign as compared to the retardation of the liquid crystal material of the reflective light modulator.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/770,846, filed Feb. 3, 2004 by the same inventors, which has issuedas U.S. Pat. No. 7,165,843 and is incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to liquid crystal display devices, andmore particularly to a method and apparatus for preventing unwantedreduction of contrast in images created thereby. A predominant currentusage of the inventive improved optical system with angular compensationis in the correction of angularly dependent error caused bybirefringence in reflective liquid crystal display devices, particularlybut not exclusively in off axis optical systems, wherein angulardependant differences in the index of refraction will tend to createunintended changes in the brightness of an image.

2. Description of the Background Art

In a liquid crystal display (LCD”) imaging apparatus, it is important tocarefully regulate the amount of light which forms each pixel of adisplayed image, so as to provide the correct amount of contrast betweenlighter and darker portions of an image.

FIG. 1. is a block diagrammatic representation showing a typicalreflective display based three color projection system 100, illustratingthe operation of a polarizing electro-optical imaging system. Theprojection system 100 has an illumination source 102, a polarizing beamsplitter 104, a color separator 106, a plurality of reflective liquidcrystal displays (LCDs) 108(r, g, and b), and projection optics 110.Illumination source 102 generates a source beam of white light anddirects the source beam toward polarizing beam splitter 104, whichpasses one portion of the source beam having a first polarity, andredirects another portion (an illumination beam) of the source beamhaving a second polarity along a system axis 112, toward color separator106. Color separator 106 separates the illumination beam into its red,green, and blue components, and directs each of these coloredillumination beams to a respective one of LCDs 108(r, g, and b). Each ofLCDs 108(r, g, and b) is controlled by a system, e.g., a computer orother video signal source (not shown), and modulates the polarity ofselective portions (i.e., pixels) of the colored illumination beams toform colored imaging beams, which are reflected back toward colorseparator 106. Color separator 106 recombines the colored imaging beamsto form a composite imaging beam and directs the composite imaging beamback along system axis 112, toward polarizing beam splitter 104, whichpasses only the modulated portions of the composite imaging beam toprojection optics 110. Projection optics 110 then focuses the modulatedportions of the composite imaging beam onto a display surface (notshown).

The example of FIG. 1 is an “on-axis” system, in that the beams from thebeam splitter 104 to the color separator 106 and also the beams from thecolor separator 106 back through the beam splitter 104 are on the commonsystem axis 112. Even in such an example, some rays of the beam willimpinge upon the LCDs 108 at different angles as compared to other rays,although the chief, or average, ray will be essentially perpendicular tothe surface of the LCDs 108. As can be appreciated by one skilled in theart, and as will be discussed in more detail hereinafter, in “off-axis”systems, wherein the two paths diverge, the chief, or average, ray willbe far from perpendicular to the surface of the LCDs 108.

Typically, in a reflective liquid crystal display apparatus such as thethree color projection system 100, each portion of light (light ray)which is to represent a pixel in a resultant image is polarized with apolarizer (such as the polarizing beam splitter 104 of FIG. 1), and isthen directed through a liquid crystal material such as is found in theLCD's 108, and then exits toward the projection optics 110 through an“analyzer”. In the “on axis” example of FIG. 1, the beam splitter 104serves both as the polarizer and as the analyzer.

There are two problems associated with the angular dependence of theretardation of the LC material. Note that any angular variation of theretardation will change the way in which the LCD modifies thepolarization of light, and therefore will change the system performance,almost always in a negative fashion. In most cases, the LC is designedto work optimally on axis, that is, for a ray of light that impingesupon the LCD imager perpendicular to its surface. In any real displaysystem, the light hitting any given point on the projection screen willcome from a whole range of angles. The farther these rays are from theperpendicular at the LCD, the further the operation of the LCD for theserays will be from optimal. While the details of the angular distributionwill depend on the details of the optical design, there will be asignificant range of angles for all such designs. In general, thisvariance from perpendicularity will degrade the performance of thesystem, most noticeably by lowering the contrast.

For the on axis design of FIG. 1, the center ray of this range of angleswill be close to perpendicular at the LCD, but even in this case, therewill be a degradation of performance because of the manynon-perpendicular rays. For the off-axis design of FIG. 2, however, thecenter ray will in general be far from perpendicular, and it is possiblethat none of the rays will be so.

Secondly, such angular change will be different in different areas ofthe liquid crystal display apparatus. As discussed above, the light fromeach pixel on the LCD, which is transferred to a corresponding point onthe projection screen, is an average over many angles. For some opticaldesigns, and particularly for the off-axis design of FIG. 2, this set ofangles will be different for each and every pixel. Therefore, theundesired contrast shift discussed above will not be uniform across theentire liquid crystal display.

It would be advantageous to have some method or means for preventingunwanted polarization aberrations across the surface of a liquid crystalimaging apparatus.

SUMMARY

The present invention overcomes the problems discussed above in relationto the prior art by providing a compensator in the beam path. Accordingto the present invention, a compensator is placed in the path of lightincident upon and/or reflected from a liquid crystal imaging device. Thecompensator is designed to exhibit retardation that is generally equalin magnitude and opposite in sign to the retardation of the underlyingliquid crystal material. In a particular embodiment, the compensator isa negative c-plate produced by form birefringence (periodic stacks ofthin film layers), and the imaging device is a vertically aligned,nematic liquid crystal device (“VAN cell”). In at least some embodimentsof the invention the compensator is placed very close to the liquidcrystal portion of the LCD display device, such that the angle ofincidence of each ray on the compensator will be very close to thecorresponding angle of incidence for the same ray on the liquid crystallayer. For example, in one system, the negative c-plate is formed on thecover glass of the imager. In another example, the negative c-plate isformed on the plano surface of a field lens.

It is an advantage of the present invention that variations in theretardation seen by each ray portion of a beam are neutralized by acompensator.

It is another advantage of the present invention that the intendedbrightness for each portion of an image is achieved.

It is still another advantage of the present invention that improvedcontrast can be achieved, since the angular dependence of the LCretardation will have been substantially compensated for.

It is still another advantage of the present invention that improvedcontrast uniformity across the display can be achieved, since there isno substantial differential brightness offset in different portions of aprojected image caused by differences in the index of refraction towhich such different portions may be subjected.

It is yet another advantage of the present invention that the desiredbrightness for each pixel is achieved, without unwanted variations oraberrations caused by the retardation of the liquid crystal displaydevice.

These and other objects and advantages of the present invention willbecome clear to those skilled in the art in view of the description ofmodes of carrying out the invention, as described herein and asillustrated in the several figures of the drawing. The objects and/oradvantages discussed herein are not intended to be an exhaustive listingof all possible objects or advantages of the invention. Moreover, itwill be possible to practice the invention even where one or more of theintended objects and/or advantages might be absent or not required inthe application.

Further, those skilled in the art will recognize that variousembodiments of the present invention may achieve one or more, but notnecessarily all, of the above described objects and/or advantages.Accordingly, any objects and/or advantages that are discussed herein arenot essential elements of the present invention, and should not beconstrued as limitations.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the followingdrawings, wherein like reference numbers denote substantially similarelements:

FIG. 1 is a block diagram of a prior art reflective projection system;

FIG. 2 is a block diagram of an off-axis projection system including acompensation element according to the present invention;

FIG. 3 is a side diagrammatic view of one channel of a multi-channelimaging system including an aberration compensation element according tothe present invention;

FIG. 4 is another diagrammatic view of the imaging system of FIG. 3;

FIG. 5 is a diagrammatic view showing a portion of an imaging systemwith a compensator element affixed to the piano surface of a lens;

FIG. 6 is a diagrammatic view showing a portion of an imaging systemwith a compensator element affixed directly to an LCD device;

FIG. 7 is a diagrammatic view showing one relative orientation of theoptical axes of a compensator element and an imager;

FIG. 8 is a diagrammatic view showing another relative orientation ofthe optical axes of a compensator element and an imager;

FIG. 9 is a diagrammatic view showing another relative orientation ofthe optical axes of a compensator element and an imager;

FIG. 10 is a diagrammatic view of a physical implementation to achievethe relative orientation of optical axes shown in FIG. 9; and

FIG. 11 is a diagrammatic view of another physical implementation toachieve the relative orientation of optical axes shown in FIG. 9.

DETAILED DESCRIPTION

This invention is described in the following description with referenceto the Figures, in which like numbers represent the same or similarelements. While this invention is described in terms of modes forachieving this invention's objectives, it will be appreciated by thoseskilled in the art that variations may be accomplished in view of theseteachings without deviating from the spirit or scope of the presentinvention. The embodiments and variations of the invention describedherein, and/or shown in the drawings, are presented by way of exampleonly and are not limiting as to the scope of the invention. Unlessotherwise specifically stated, individual aspects and components of theinvention may be omitted or modified, or may have substituted thereforeknown equivalents, or as yet unknown substitutes such as may bedeveloped in the future or such as may be found to be acceptablesubstitutes in the future. The invention may also be modified for avariety of applications while remaining within the spirit and scope ofthe claimed invention, since the range of potential applications isgreat, and since it is intended that the present invention be adaptableto many such variations.

In the following example, some details of some well known opticalpractices (such as alignment, mounting, focusing, and the like) andcomponents (such as illumination sources, various lenses, reflective LCDimagers, and the like) have been omitted, so as not to unnecessarilyobscure the present invention.

The present invention overcomes the problems associated with the priorart, by providing an electro-optical imaging system having an aberrationcompensation element disposed between a polarizer and an analyzer tocompensate for polarization aberrations resulting from variations of theretardation of the LCD imaging device with ray incident angle.

FIG. 2 is a diagrammatic representation of an example of an off-axisprojection system 200 to having an illumination source 202, a condenserlens 204, a polarizer 206, a reflective LCD 208, an analyzer 210, and aprojection lens group 212. The illumination source 202 generates anillumination beam 214 that is focused by condenser lens 204 to passthrough polarizer 206, and impinge on LCD 208 at a non-perpendicularangle. LCD 208 modulates the polarized illumination beam 214, on a pixelby pixel basis, to form an imaging beam 216, and reflects the imagingbeam 216 through analyzer 210 toward projection lens group 212. A systemaxis 218 is perpendicular to the plane of the LCD 208.

In this present example, the angular separation between illuminationbeam 214 and imaging beam 216 allows for the separation of polarizer 206and analyzer 210. In this particular embodiment, polarizer 204 andanalyzer 210 are both sheet polarizers of material HN42HE manufacturedand sold by Polaroid Corporation. The transmission axes of polarizer 206and analyzer 210 are oriented orthogonal to one another, such that nolight will pass through analyzer 210, unless the polarization vector ofa particular light ray is modulated by reflective LCD 208, or unless thelight is subject to a polarization aberration. If, however, a pixel ofreflective LCD 208 rotates the polarization axis of a light ray by 90degrees, then the light should pass through analyzer 210 at maximumintensity, unless the light is subjected to a polarization aberration. Apolarization state that is intermediate between these two extreme casesresults in an intermediate intensity level.

As can be seen in the view of FIG. 2, a compensation element 222 isplaced in front of the reflective LCD 108 such that the illuminationbeam 214 and the imaging beam 216 both pass there through. As will bediscussed in greater detail hereinafter, the compensation element 222 isa negative c plate which has retardation approximately equal inmagnitude but opposite in sign as the retardation of the liquid crystalmaterial in the reflective LCD 208.

In anisotropic materials, the index of refraction (n) of the materialdepends on the direction that the light is traveling. Anisotropicmaterials are, therefore, also referred to as being birefringent.

Nematic liquid crystals are a type of uniaxial material. Uniaxialmaterials are birefringent materials that have a unique direction,referred to as the optical axis, along which the index of refraction(n_(e)) is an extremum (either maximum or minimum). For all raystraveling perpendicular to the optical axis the index of refraction(n_(o)) is constant. The birefringence (Δn) of uniaxial materials isdefined as the difference between the index of refraction along theoptical axis (n_(e)) and the index of refraction along a directionperpendicular to the optical axis (n_(o)):Δn=n _(e) −n _(o)Note that the birefringence (Δn) can be either a positive or negativevalue, and is a property of the material not dependent on angle ofincidence.

Polarization aberrations are induced by birefringent materials as aresult of retardation. Retardation (R) is the total phase differenceinduced in two mutually perpendicular polarized states of light as aresult of the different indices of refraction n_(e) and n_(o).Retardation can be expressed as a distance:R=Δn·dwhere (d) is the thickness of the material traversed. Note that Δn, asdescribed above, is unitless.

In order for compensation element 222 to compensate for polarizationchanges caused by the retardation of LCD 208, the retardation induced bycompensation element 222 should be equal in magnitude, but opposite insign to the retardation induced by LCD 208. That is, the retardation ofthe compensation element 222 should conform generally to the equation:Δn _(lc) ·d _(lc) +Δn _(c) ·d _(c)=0where (Δn_(lc)) is the birefringence of the liquid crystal material ofthe LCD 208, (d_(lc)) is the thickness of the liquid crystal layer;(Δn_(c)) is the birefringence of compensation element 222, and (d_(c))is the thickness of compensation element 222. As will be discussed ingreater detail hereinafter, it is desirable to keep the compensationelement 222 quite close to the reflective LCD 108 such that the angle ofthe incident light ray will not be substantially different as betweenthe compensation element 222 and the reflective LCD 108. Further, it isdesirable that the liquid crystal layer of LCD 208 and compensationelement 222 have the same angular dependence of retardation.

To obtain similar angular dependence, the liquid crystal layer of LCD208 and compensation element 222 are selected to be c-plates. Mostuniaxial devices and/or compensation elements can be classified aseither an “a-plate” or a “c-plate.” An a-plate is a uniaxial materialoriented with the optical axis parallel to the incident surface of thematerial. Therefore, a ray of light incident on an a-plate on-axis(perpendicular to the surface of the material) will be subjected to thefull retardation. In contrast, a c-plate is a uniaxial material orientedwith the optical axis perpendicular to the incident surface of thematerial. Therefore, a ray of light incident on a c-plate on-axis willbe subjected to no retardation, because the index of refraction for alldirections perpendicular to the optical axis is the same (i.e., n_(o)).Further, similarly oriented c-plates will exhibit similar angulardependence with respect to retardation.

Most compensation elements (e.g., quarter-wave plates, half-wave plates,etc.) are a-plates. However, VAN cells are essentially c-plates, and theinventors have found that using negative c-plate compensator 222 incombination with LCD 208 (a VAN cell) significantly corrects for angulardependent polarization aberrations. Compensation element 222 is anegative c-plate, because VAN cells are positive c-plates and, asindicated above, the retardation of compensation element 222 must beequal in magnitude but opposite in direction compared to LCD 208.

The invention can be described in a much more general way. For a givenoptical system containing one or more LCDs, each of the LCDs has beendesigned to work optimally for some given voltage state, characterizedby a voltage V_(opt), and for some given ray angle, characterized by aviewing angle Ω_(opt). In many cases, the optimal voltage willcorrespond to that which produces the dark state of the system, sincethis is frequently that state which is most susceptible to performancedegradation due to birefringence.

In general, the retardation of the LC will depend on both the voltage Vand the viewing angle Ω. In the optimal state, the retardation willequal the specific optimal retardation value, defined by valueR_(LCD)(Ω_(opt))=R_(opt). For other angles Ω, the LCD retardation willhave other values. Because, as described above, the light reachingscreen 220 in FIG. 2 is an average over many angles Ω, the performanceof the system will be degraded because the retardation of the LC is notequal to the value R_(opt) for all angles Ω.

The invention comprises adding a compensation means to the system suchthat the sum of the retardations of the LCD and the compensation meansare equal to R_(opt), for all angles Ω which contribute to the systemperformance at the screen. That is,R _(LCD)(Ω)+R _(C)(Ω)=R _(opt),for all Ω which contribute to system performance at the screen.

The invention can be further generalized in two ways. First, there maybe other elements in the system which contribute to overall retardation,for example, polarizers or thin film coatings on various opticalelements. Assuming that there are N such elements and each of these havetheir own retardation R_(n)(Ω), then optimum compensation is achievedwhen

${{R_{LCD}(\Omega)} + {R_{C}(\Omega)} + {\sum\limits_{n = 1}^{N}{R_{n}(\Omega)}}} = R_{opt}$

That is, the compensation means is compensating for all birefringentelements in the system.

Secondly, there may be systems where the angles Ω are different whenpassing through each of the different elements. For example, if there isan optical element with power situated between the LCD and thecompensating means, then Ω_(LCD) and Ω_(C) will be different for aparticular ray. In this case, the optimization requirement for aparticular ray can be expressed as follows:

${{{R_{LCD}\left( \Omega_{LCD} \right)} + {R_{C}\left( \Omega_{C} \right)} + {\sum\limits_{n = 1}^{N}{R_{n}\left( \Omega_{n} \right)}}} = R_{opt}},$where now the ray impinges on each element at a different angle.

It will be clear to those skilled in the art that the above conditionsdefining the retardation value of the compensation element will offer agreat benefit to the system performance even when the conditions are notcompletely met, which will often be the case in a physical embodiment.

An example of such a non-optimal but still beneficial invention willoccur for most real systems. Referencing FIG. 2, consider a point 232 onthe screen 220. The light which impinges on this point determines thesystem performance at that point. As indicated, all of the light withinthe ray bundle 231 hits the screen at this point. This ray bundle iscentered around the chief ray 230. Note that the angles Ω are differentfor the various rays in the ray bundle. In many systems it will not bepossible to have the optimization condition defined above to be metprecisely for each and every ray in the ray bundle 231. One approach tothis problem is to meet this condition exactly for the chief ray 230.Alternatively, some average of the above optimization condition can beminimized over the entire ray bundle 231. Both of these embodiments ofthe invention should lead to improved system performance, e.g., improvedcontrast.

In reference to the specific embodiment described above, the VAN cell isusually optimized for the voltage V_(opt)=0 (because this is the darkstate), and for the on-axis viewing angle. In this case, the optimalretardation is equal to or close to 0. However, other applicationsshould be clear to those skilled in the art. For example, the LCD couldbe an ECB (Electronically Controlled Birefringence) or Pi cell, in whichcase the optimal voltage would not be 0, but some higher value, forexample 5 V, since the dark state for these modes occurs at this highervoltage. Other than this change in voltage, the invention still appliesdirectly.

For the specific embodiment using a VAN cell, the way in which theinvention can be realized is as follows. The optimal voltage is 0 V, andthe optimal angle is on-axis. For this optimal state, the VAN cell formsa c-plate with positive birefringence, and its on-axis (optimal)retardation is zero. The invention requires that the sum of the LCD andcompensation element retardations sum to the optimal value of zero forall angles Ω. This can be achieved by requiring the compensating elementto be a c-plate with negative birefringence, since in this case theretardations of the two will sum to the optimal value of zero for allangles Ω.

In the embodiment of the invention described, the negative c-plate(compensation element 222) is constructed using a technique known as“form birefringence”. This technique is described in the publicationPrinciples of Optics by Born and Wolf, which is well known and commonlyavailable. According to the form birefringence method, the negativec-plate is formed by building up a periodic stack of thin film layers.

FIG. 3 shows one channel (the green channel) of a multi-channel(multi-color) off-axis projection system 300, including an illuminationsource 302, a pair of dichroic plates 304 (r and b), a polarizer 306(g),a compensation element 308(g), a field lens 310(g), a reflective LCD312(g), an analyzer 314(g), and a projection lens group 316.Illumination source 302 generates an illumination beam 318, and directsillumination beam 318, along an optical path 320, through dichroicplates 304(r, b) polarizer 306(g), aberration compensation element308(g), and field lens 310 to impinge on LCD 312(g). Dichroic plates 304(r and b) reflect red and blue portions (not shown in FIG. 3) ofillumination beam 318, respectively, and transmit the green portion318(g) of illumination beam 318. Polarizer 306(g) linearly polarizesgreen illumination beam 318(g) into a first polarized state,corresponding to the transmission axis of polarizer 306(g). LCD 312(g)modulates the polarized beam and reflects an imaging beam 322(g) (themodulated, polarized beam) through analyzer 314(g).

FIG. 4 is another diagrammatic view of the projection system 300 of FIG.3 showing the green channel in context with the remaining color channelsof the system. In this view, system axis 330 lies in the plane of thepage, while imaging beams 322 (r, g, b) and 322 extend up out of thepage, and obscure the views of illumination beams 318 (r, g, b) and 318,which rise from illumination source 302 located beneath the plane of thepage. Similarly, analyzers 314 (r, g, b) are disposed above the plane ofthe page, and obscure the views of polarizers 306 (r, g, b),respectively. Compensation elements 308 (r, g, b) and imagers 312 (r, g,b) pass through the plane of the page and are centered on system axis330.

The LCD 312(g) is controlled by a system, such as a computer or videosignal source (not shown), such that the polarity of selected portions(i.e., pixels) of green illumination beam 318(g) are modulated to form agreen imaging beam 322(g), which is reflected along an optical path 324,through analyzer 314(g) and dichroic plates 304(r, b). Dichroic plates304 (r, b) combine green imaging beam 322(g) with the red and blueimaging beams 322(r) and 322(b) (which have also been modulated asdescribed), to form imaging beam 322 which continues along optical path324 into projection lens group 316. Field lens 310(g) focuses theaperture stop (not shown) of illumination source 302 at a field stop(not shown) near the rear of projection lens group 316, thus avoidingthe loss of much of the light of illumination beam 318.

Analyzer 314(g) is also a linear polarizer. Projection system 300 canoperate in at least two different modes. For example, if thetransmission axis of analyzer 314(g) is oriented parallel to thetransmission axis of polarizer 306(g), then analyzer 314(g) will passunmodulated portions and block modulated portions of green imaging beam322(g). On the other hand, if the transmission axis of analyzer 314(g)is oriented orthogonally with respect to the transmission axis ofpolarizer 306(g), then analyzer 314(g) will pass modulated portions andblock unmodulated portions of green imaging beam 322(g). In oneembodiment, polarizer 306(g) and analyzer 314(g) are both fashioned fromHN42HE polarizing material available from Polaroid Corporation.

The compensation element 308(g) is positioned in green illumination beam318(g) and imaging beam 322(g), to compensate for retardation induced inthe beams due to the birefringent property of the reflective LCD 312(g).The use of separate compensation elements for each color of theprojection system 300 permits the compensation elements 308 to beoptimized for a particular color (red, green or blue) of light. Alsonote that the compensation elements 308 will work most effectively ifthere is no optical element with optical power between these and theLCDs 312.

FIGS. 5 and 6 are side diagrammatic views showing examples of thepolarizer 306, the analyzer 314, and the reflective LCD 312, aspreviously discussed herein. In the view of FIG. 5 it can be seen thatthe compensation element 308 is positioned on the back of the field lens310. In the view of FIG. 6, the compensation element 308 is positioneddirectly on the front glass of the reflective LCD 312. It isadvantageous that rays of light which pass through the liquid crystallayer of the reflective LCD 312 pass through compensation element 308 atthe same, or nearly the same, angle. This can be accomplished by placingthe compensation element 308 on the plano surface of a lens (theoptional field lens 310 in this example). Alternatively, thecompensation element 308 can be positioned on essentially any surface(such as another type of lens, an optically inactive substrate, or thelike) which is positioned relatively close to the surface of thereflective LCD 312. As illustrated by the example of FIG. 6, thecompensation element 308 can also be formed or placed directly on thecover glass of the reflective LCD 312.

In general, the requirement for the compensation element 308 to functionoptimally is that there be no optical element with optical power betweenit and LCD 312. However, compensation element 308 can still provide asignificant improvement, even if not optimal, when optical elements withpower are disposed between it and LCD 312. Thus, it is not an essentialelement of the invention that absolutely no optical elements with powerbe disposed between compensation element 308 and LCD 312.

It should be noted that, when the compensation element 308 is createdusing “form birefringence” method discussed above, the thin films usedmay be made from inorganic materials. In such instances, thecompensation element 308 can be made very rugged, and will have a verylong useful life.

It should also be noted that the invention is intended to correct forretardation of light modulation elements which have a known ordiscernable birefringence characteristic, and for which a compensationelement can be formed using known methods or those yet to be developed.

FIG. 7 is a diagrammatic view showing one relative orientation of theoptical axes of a compensation element 308 and the liquid crystal layerof an imager 312. In particular, the optical axis n_(ec) of compensationelement 308 is perfectly aligned with the optical axis n_(elc) of imager312. Additionally, optical axis n_(ec) is oriented perpendicular to theincident surfaces 702 of compensation element 308, and optical axisn_(elc) is oriented perpendicular to the incident surfaces 704 of theliquid crystal layer of imager 312. Thus, compensation element 308 andimager 312 are true c-plates. While this particular orientation of theoptical axes n_(ec) and n_(elc) would provide the desired correspondencebetween the angular dependency of the retardation of compensationelement 308 and imager 312, it is more likely that the orientation ofoptical axis n_(ec) and/or n_(elc) will be slightly modified toaccommodate practical design considerations.

FIG. 8 is a diagrammatic view showing one such modification of therelative orientation of the optical axis n_(ec) of compensation element308 and n_(elc) of an alternate imager 312A. Note that optical axisn_(elc) is tilted slightly with respect to the vertical, and is nolonger perfectly aligned with optical axis n_(ec) of compensationelement 308. Nevertheless, this embodiment of the invention provides asignificant improvement over the prior art.

The tilting (e.g., 3-5 degrees) of optical axis n_(elc) is typicallydesigned into VAN cells, so that when an electric field is appliedacross the liquid crystal there is a preferred direction in which thecrystals will uniformly “fall over.” Thus, in this example, imager 312Ais not technically a true c-plate, because optical axis n_(elc) is notperpendicular to incident surfaces 704. However, as used herein the termc-plate should be understood to include such materials wherein theoptical axis is substantially perpendicular to the surface of thematerial. According to this definition, imager 312A is a c-plate. Asused herein, the term substantially perpendicular includes slightdeviations from the true perpendicular by as much as 20 degrees,although smaller deviations (e.g., 3-10 degrees) will be more common inmost applications.

FIG. 9 is a diagrammatic view showing another relative orientation ofthe optical axes n_(ec) and n_(elc). In this particular example, opticalaxis n_(ec) of an alternate compensation element 308A is also titled, soas to be parallel with optical axis n_(elc) of imager 312A. While thealignment of optical axes n_(ec) and n_(elc) in this example is expectedto provide some improvement as compared to the example of FIG. 8, theinventors expect that compensation element 308A might be difficult tomanufacture.

FIG. 10 is a diagrammatic view of a physical implementation to achievethe relative orientation of optical axes n_(ec) and n_(elc), as shown inFIG. 9, but using compensation element 308 of FIG. 7 and FIG. 8, whichcan be manufactured via the “form birefringence” method described above.In particular, compensation element 308 is physically tilted to alignoptical axes n_(ec) and n_(elc) (not shown in FIG. 10). For example, asshown in FIG. 10, compensation element 308 is positioned with respect tofield lens 310 via a wedge 350. As another example, as shown in FIG. 11,compensation element 308 is positioned with respect to imager 312 via awedge 352. Alternatively, compensation element 308 can be separate fromboth field lens 310 and imager 312, and be independently mounted toalign optical axes n_(ec) and n_(elc) (not shown in FIG. 10).

The invention has been described herein primarily using an off-axis(non-telecentric) system. It is thought that this type of system willbenefit most from the present invention, since the greater the angle ofincidence between the LCD imagers and the beams entering and leaving theliquid crystal material thereof, the greater will be the unwantedeffects of the birefringence property of the material and, therefore,the greater will be the need to compensate for such effects. However, itshould be noted that, even in an on-axis system (such as the prior artexample shown in FIG. 1), there will be some difference in the range ofangles of incidence of rays entering and leaving the LCD material.Therefore, it is thought that the invention will also provide asignificant advantage in an on-axis system, also, in at least someapplications. Further, it is anticipated that the invention will providea significant improvement when used in conjunction with transmissivedisplays.

In some cases, there may be other optical elements in the display systemwhich have birefringence, and in these cases, the retardation of thecompensator can be modified to compensate for the angular dependence ofthe LCD and these other elements. For example, it is not uncommon forthe polarizer and analyzer elements to have components which areessentially c-plates. In this case, the compensator retardation shouldbe set to be equal in magnitude but opposite in sign as the sum total ofall c-plates in the system, which includes the LCD and the polarizer andanalyzer.

All of the above are only some of the examples of available embodimentsof the present invention. Those skilled in the art will readily observethat numerous other modifications and alterations may be made. Many ofthe described features may be substituted, altered or omitted withoutdeparting from the spirit and scope of the invention. Therefore, oneskill in the art could readily create variations of the invention toadapt it according to the needs or convenience of a particularapplication. Accordingly, this disclosure is not intended as limitingand the appended claims are to be interpreted as encompassing the entirescope of the invention.

1. An electro-optical imaging system comprising: a polarizer disposed topolarize an illumination beam to form a polarized illumination beam; areflective light modulator for modulating said polarized illuminationbeam to form a modulated beam; an analyzer disposed in the path of saidimaging beam; and a compensation element for compensating foraberrations dependent on the angles at which the illumination beam andthe modulated beam enter and leave the reflective light modulator; andwherein said light modulator exhibits a particular retardation value ata particular angle of incidence responsive to the application of aparticular voltage; and said compensation element has a retardationvalue that is dependent on said particular retardation value of saidlight modulator.
 2. An electro-optical imaging system according to claim1, wherein said particular voltage is a non-zero voltage.
 3. Anelectro-optical imaging system according to claim 2, further comprising:an additional optical component having a retardation disposed in atleast one of said illumination beam and said modulated beam; and whereinthe retardation of said compensation element is roughly equal inmagnitude but opposite in sign as the sum of the retardations of saidlight modulator at said non-zero voltage and said additional opticalcomponent.
 4. An electro-optical imaging system according to claim 2,wherein the sum of the retardation of said compensation element and theretardation of said light modulator at angles other than said particularangle of incidence is roughly equal in magnitude as the particularretardation value of said light modulator at said non-zero voltage. 5.An electro-optical imaging system according to claim 4, wherein saidnon-zero voltage corresponds to a dark-state of said light modulator. 6.An electro-optical imaging system according to claim 2, furthercomprising: at least one additional birefringent component; and whereinthe sum of the retardation of said compensation element, the retardationof said at least one additional birefringent element, and theretardation of said light modulator at angles other than said particularangle is roughly equal to the particular retardation value of said lightmodulator at said non-zero voltage.
 7. An electro-optical imaging systemaccording to claim 2, further comprising: at least one optical elementwith power disposed between said light modulator and said compensationelement, such that a light ray impinging on said light modulator at afirst angle will impinge on said compensation element at a seconddifferent angle; and wherein the retardation of said compensationelement for rays impinging on said compensation element at said secondangle is roughly equal in magnitude but opposite in sign as theretardation value of said light modulator for rays impinging on saidlight modulator at said first angle.
 8. An electro-optical imagingsystem according to claim 2, further comprising: at least one opticalelement with power disposed between said light modulator and saidcompensation element; and wherein the sum of the retardation of saidcompensation element, the retardation of said at least one opticalelement with power, and the retardation of said light modulator atangles other than said particular angle is roughly equal in magnitude tothe particular retardation value of said light modulator at saidnon-zero voltage.
 9. An electro-optical imaging system according toclaim 1, wherein: said light modulator has an optical axis passingthrough the surface of said light modulator; said compensation elementhas an optical axis substantially perpendicular to said surface of saidcompensation element; said optical axis of said light modulator istilted with respect to a line perpendicular to the surface of said lightmodulator; and said compensation element is tilted with respect to saidlight modulator.
 10. An electro-optical imaging system according toclaim 9, wherein the optical axis of said compensation element isaligned with respect to the optical axis of said light modulator.
 11. Anelectro-optical imaging system according to claim 9, wherein the opticalaxis of said compensation element is parallel to the optical axis ofsaid light modulator.
 12. An electro-optical imaging system according toclaim 9, wherein said particular voltage is a non-zero voltage.
 13. Anelectro-optical imaging system according to claim 1, wherein said lightmodulator is a vertically-aligned, nematic liquid crystal cell.
 14. Anelectro-optical imaging system according to claim 1, wherein: saidreflective light modulator is a liquid crystal display; and saidcompensation element is a negative c-compensator.
 15. An electro-opticalimaging system according to claim 1, wherein said compensation elementhas a retardation value approximately equal in magnitude but opposite insign to said particular retardation value of said light modulator. 16.An electro-optical imaging system according to claim 1, wherein saidcompensation element is disposed on a surface of said light modulator.17. An electro-optical imaging system according to claim 1, furthercomprising: a field lens proximate said light modulator; and whereinsaid compensation element is disposed on a surface of said field lens.18. A projection system comprising: a color separator disposed toseparate a multi-colored illumination beam into a plurality of coloredillumination beams; a plurality of polarizers, each polarizer disposedto polarize a respective one of said colored illumination beams to forma polarized, colored illumination beam; a plurality of reflective lightmodulators, each reflective light modulator for modulating a respectiveone of said polarized, colored illumination beams to form a coloredimaging beam, each of said reflective light modulators disposed toreceive a respective one of said polarized, colored illumination beamsand further to reflect said colored imaging beam; a plurality ofanalyzers, each of said analyzers disposed to analyze a respective oneof said modulated colored beams; a plurality of compensation elementsfor negating effects of retardation of the reflective light modulators,each of said compensation elements disposed between a respective one ofsaid polarizers and a respective one of said analyzers; and a colorcombiner disposed to combine said colored imaging beams to form amulti-colored imaging beam; and wherein each of said plurality of lightmodulators exhibits a particular retardation value at a particular angleof incidence responsive to the application of a particular voltage; andeach of said compensation elements associated with a respective one ofsaid light modulators has a retardation value that is dependent on saidparticular retardation value of said respective light modulator.
 19. Aprojection system according to claim 18, wherein said particular voltageapplied to each of said plurality of light modulators is a non-zerovoltage.
 20. A projection system according to claim 19, wherein saidparticular voltage applied to at least one of said plurality of lightmodulators is different than said particular voltage applied another oneof said plurality of light modulators.